Until now, driver assistance techniques have been intended to provide assistance, such as warning deviation from a lane or keeping the speed of a vehicle constant, to drivers. However, in recent years, with increase in awareness of safety, and advancement in performance and price reduction of processors or various sensor devices, driver assistance techniques relating to active safety, such as emergency automatic braking or emergency steering avoidance, for avoiding collision with obstacles have been commercialized, researched, and developed. In driver assistance techniques for avoiding collision with obstacles, it is necessary to properly perceive risk of collision with obstacles. Techniques of setting a risk map indicating such risk depending on the relative speed between an own vehicle and an obstacle have been devised (for example, Patent Literature 1).
The risk map indicates a range within which the obstacle can exist after one unit of time and degree of risk when the own vehicle enters the range. The own vehicle can prevent collision with the obstacle after one unit of time by traveling while trying not to enter the range. The collision risk calculation device described in Patent Literature 1 determines a range of a risk map of a front obstacle depending on the relative speed of an own vehicle relative to the speed of the front obstacle. Thus, the range of the risk map is uniformly increased when the speed of the front obstacle is lower than the speed of the own vehicle, and uniformly decreased when the speed of the front obstacle is higher than the speed of the own vehicle. However, when the range of the risk map is uniformly determined depending on the relative speed, the risk of collision cannot be accurately estimated. For example, when the speed of the front obstacle is lower than the speed of the own vehicle, since it is predicted that the own vehicle is closer to the front obstacle after a predetermined time period, the range of the risk map is set to be large. Thus, the own vehicle largely avoids and overtakes the obstacle or brakes suddenly so as not to enter in a direction of the risk map, which is set to be large. However, actually, when the speed of the front vehicle is significantly high, the front vehicle is less likely to turn right or left without slowing down.
As such, the collision risk calculation device described in Patent Literature 1 uniformly sets the range of the risk map to be large in left and right directions depending on the relative speed of the own vehicle relative to the speed of the front obstacle even when the speed of the front vehicle is significantly high, and thus may cause unnecessary actions, such as an action of largely avoiding an obstacle to the own vehicle or sudden braking.